This invention relates to a mixture of isomers of 2,2'-azobis(2,4-dimethylvaleronitrile) (hereinafter referred to as "ADVN") having greater solvent solubility than an isomer of ADVN having a low melting point.
ADVN is useful as a radical polymerization initiator, blowing agent and the like. It is particularly useful as polymerization initiator for vinyl chloride and acrylonitrile and also widely useful as polymerization initiator for polymerizable unsaturated monomers such as ethylene, styrene, acrylic acid esters, methacrylic acid esters, vinyl fluoride, vinyl acetate, acrylamide, acrylic acid, methacrylic acid, vinyl pyridine, and the like.
It is known that ADVN has two isomers, one of which is a meso-form having a melting point as low as 56.degree. C. to 57.degree. C., i.e. a low melting point isomer (hereinafter referred to as "LMP"). The other isomer is a dl-form having a melting point as high as 74.degree. C. to 77.degree. C., i.e. a high melting point isomer (hereinafter referred to as "HMP") as reported in J. Am. Chem. Soc., vol. 73, pages 2618-2621 (1951) by C. G. Overberger.
The mixing ratio of the two isomers can be controlled to some extent by selecting manufacturing methods in industrial production (Japanese Patent Appln Kokai (Laid-Open) No. 131827/76).
Commercially available ADVN is LMP only or HMP only or a mixture of HMP and LMP in about a 1:1 mixing ratio (by weight).
As to solvent solubility of the two isomers of ADVN, it is known that the solubility of LMP is several times higher than that of HMP and of a commercially available 1:1 mixture of LMP and HMP. Thus, it has been believed that if ADVN containing only LMP can be produced, this ADVN would present advantages over the conventional mixture in terms of polymerization activity and selection of solvent used for polymerization; these properties are very important for a new polymerization initiator.
The present inventors have studied the solvent solubility of ADVN containing LMP and HMP in various mixing ratios, investigated undesirable influences on the solvent solubility of ADVN, find that a special mixture of HMP and LMP has higher solvent solubility than LMP alone and accomplished this invention.